Method for reducing residual vinyl
acetate monomer

ABSTRACT

IN THE PROCESS OF COPOLYMERIZING VINYL ACETATE AND ETHYLENE UNDER LOW PRESSURE OF 100-3000 P.S.I. THE RESIDUAL VINYL ACETATE MONOMER AFTER THE MAIN COPOLYMERIZATION IS LOWERED TO LESS THAN 0.35 PERCENT BY REDUCING THE PRESSURE (TO ATMOSPHERIC ) AND PROVIDING FREE RADICALS TO THE SYSTEM CONTAINING THE RESIDUAL MONOMER. THE FREE RADICALS ARE PREFERABLY PROVIDED FROM A CHEMICAL INITIATOR SUCH AS PEROXIDE.

United States Patent Oflice Re. 28,065 Reissued July 2, 1974 28,065METHOD FOR REDUCING RESIDUAL VINYL ACETATE MONOMER AleksanderBeresniewicz and William Haworth Todd, Wilmington, Del., assignors to E.I. du Pont de Nemours and Company, Wilmington, Del.

No Drawing. Original No. 3,534,009, dated Oct. 13, 1970, Ser. No.795,057, Jan. 29, 1969. Application for reissue Oct. 13, 1972, Ser. No.297,037

Int. Cl. C08f 1/96 U.S. Cl. 260-873 17 Claims Matter enclosed in heavybrackets [II appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE In the process of copolymerizing vinylacetate and ethylene under low pressures of 100-3000 p.s.i., theresidual vinyl acetate monomer after the main copolym erization islowered to less than 0.35 percent by reducing the pressure (toatmospheric) and providing free radicals to the system containing theresidual monomer. The free radicals are preferably provided from achemical initiator such as a peroxide.

BACKGROUND OF THE INVENTION Available methods for the batchcopolymerization of vinyl acetate and ethylene to form copolymers with50-95 percent vinyl acetate usually involve two steps; a mainfree-radical initiated polymerization step wherein a major portion ofthe vinyl acetate to be included in the copolymer is copolymerized withethylene under ethylene pressures of about 100-3000 p.s.i., and afinishing step wherein residual vinyl acetate monomer is removed fromthe system containing the prepared copolymer. The finishing step isordinarily accomplished by physically removing the monomer, e.g., bysteaming or a nitrogen purge, or by chemically reacting the residualmonomer, e.g., continuing the main polymerization until the residualmonomer is exhausted. Physical removal of the residual monomer iscumbersome since additional equipment and processing steps are required.On the other hand, continuing the main polymerization is undesirablesince an extraordinarily long polymerization time is required to reducethe residual monomer to an acceptable level i.e., less than about 0.35weight percent, based on the copolymer]. In this respect, it is notunusual for the finishing step to take just as long as the mainpolymerization. Thus, heretofore, there has been no way to removeresidual vinyl acetate monomer from the vinyl acetate/ethylenepolymerization system in a completely satisfactory manner.

SUMMARY OF THE INVENTION Now, however, there is provided an improvedmethod for accomplishing the finishing step in the process ofcopolymerizing vinyl acetate and ethylene. According to the presentinvention, the finishing step is accomplished by a process whichcomprises reducing the pressure in the system containing the vinylacetate/ethylene copolymer and the residual monomer to a valuesubstantially below that at which the main polymerization isaccomplished and, while under such reduced pressure, providingsuificient polymerization initiating free radicals to the system toreduce the residual vinyl acetate monomer content below the level at thetermination of the main copolymerizatt'on step [to an amount of lessthan 0.35 percent, based on the weight of the vinyl acetate/ethylenecopolymer in the system]. The present finishing process is especiallysuitable to systems wherein the vinyl acetate/ethylene copolymer is inthe form of small particles dispersed in an aqueous medium, i.e., thesystem is a vinyl acetate/ethylene copolymer dispersion. Among otherapplications, such dispersions are generally useful as paints, papercoatings, and adhesives where residual monomer content and its attendantodor is particularly obnoxious.

DESCRIPTION OF PREFERRED EMBODIMENTS Many patents and publicationsdescribe processes for preparing vinyl acetate-ethylene copolymers.Among others, the patents include US. Pats. 3,337,482 (Watanabe) and3,355,322 (Worrall) as well as the following foreign patents: German1,127,085 (Hoechst) and 1,226,382 (Badische); and Netherlands 66,04289(Cumberland Chem). In general, the known prior art processes are usefulin preparing the vinyl acetate/ethylene copolymer systems which can besubsequently finished by the process of the present invention. Thecopolymers prepared by known processes usually contain 50-95 weightpercent copolymerized vinyl acetate; the process for their preparationordinarily involving free-radical initiated copolymerization at atemperature of 30-150 C. and a pressure of -3000 p.s.i. Also, while mostof the known processes are concerned with aqueous emulsionpolymerization systems, the finishing step of the present invention isequally applicable to vinyl acetate/ethylene copolymers prepared insolutions.

Since the copolymerization of vinyl acetate and ethylene is exothermic,a substantial cessation of the reaction exotherm indicates thetermination of the main polymerization step. Ordinarily, at this timeabout 3 to as much as about 8 weight percent residual vinyl acetatemonomer, based on the copolymer weight and determined by titration withbromine, remains in the system. It is at this time that the finishingste is accomplished. The finishing step of this invention involves, asan initial operation, reducing the pressure in the system containing thecopolymer prepared in the main step. Any pressure reduction below thatof the main polymerization permits easier removal of residual monomer.However, the finishing process becomes more efficient, i.e., takesprogressively less time to reach a given monomer level, as the magnitudeof the pressure reduction increases. Ordinarily, the pressure reductionis substantially below, i.e., to about 50 p.s.i., that of the maincopolymerization and, preferably, to that of the atmosphere. It vacuumequipment is available, greater pressure reductions can be used thoughno added advantages are thought to result.

In accomplishing the pressure reduction of the finishing step, it isparticularly advantageous to agitate the system, e.g., stirring duringpressure let down or by transferring the system from the polymerizingvessel to another vessel at lower pressure. With sutficient agitation,it is possible to blow out some of the residual monomer thus shorteningthe total time necessary for the finishing step.

In addition to the above-discussed pressure reduction, the finishingstep of the present invention includes providing sufficientpolymerization initiating free radicals to the system, while it is inthe reduced pressure state, to reduce the residual vinyl acetate monomercontent below the level at the termination of the main copolymerizationstep [to an amount of less than 0.35 percent, based on the weight of thecopolymer in the system. Preferably, the reduction is to less than 0.25percent]. While not known with certainty, it is thought that residualvinyl acetate monomer reduction occurs either by grafting orhomopolymerization. If sufficient dissolved ethylene remains,copolymerization can also occur. The essential factor with respect tothe free radicals is that they be present when the system is under apressure lower than that at which main polymerization is accomplished.

Ordinarily, the finishing step of the present invention is accomplishedimmediately after the main polymerization step. However, it can bedelayed for any amount of time by simply not providing the system withsufficient free radicals until it is desired to finish thepolymerization. Thus, polymerization finishing according to the presentinvention can be accomplished by providing the system withpolymerization initiating free radicals at any time after the mainpolymerization and the pressure reduction.

The source providing the polymerization initiating free radicals is notespecially important. Free radicals can be provided from chemicalinitiators or through irradiative techniques. With respect to chemicalinitiators, any of those known in the art can be used. Reference is madeto the text "Fundamental Principles of Polymerization by G. F. D'Alelio(John Wiley & Sons, Inc., New York 1952) for a description of manyuseful initiators. Among others, useful initiators include peroxidessuch as hydrogen peroxide, benzoyl peroxide, halogen substituted benzoylperoxides, tertiary butyl hydroperoxide, etc.; and persulfates andperborates such as the ammonium, sodium, or potassium salts.

The creation of the polymerization initiating free radicals useful inthe present process from the above initiators, i.e., activation of theinitiators, can be accomplished by any of the well-known techniques. Theinitiators can be activated either by temperature, e.g., withpersulfates at about 80 C., or through the action of a reducing agent.This latter method, customarily termed redox, is particularly suitablefor use in the present invention since generation of the free radicalsoccurs at low temperatures, for example at room temperature or evenbelow. Many suitable reducing agents for use in a redox method are setforth in U.S. Pat. 2,703,794 (Roedel) and include, among others,bisulfites, sulfoxylates, ferrous salts and tertiary aromatic amines. Ahydrogen peroxide-zinc formaldehyde sulfoxylate redox system isespecially preferred.

The manner in which the chemical initiators which provide thepolymerization initiating free radicals are introduced into the systemis not particularly important. For example, if excess quantities ofinitiators are present in the system during the main polymerization,some of the initiator may be present in the system after the pressurereduction. The residual vinyl acetate monomer can then be removed byactivating the remaining initiator. On the other hand, initiator can beadded to the reaction system after the ethylene pressure has beenreduced.

Free radicals must be provided to the system in an amount sufficient toreduce the residual vinyl acetate monomer content below the level at thetermination of the main copolymerizan'on step [to below 0.35 weightpercent]. The appropriate quantity of initiator which must be present toprovide the necessary free radicals depends on the amount of residualmonomer to be removed [remoxed] as well as the finishing conditions. Fora give residual monomer level, finishing at high temperatures e.g.,above 90 C. with an initiator such as a persulfate or above 30 C. with aredox system, requires more initiator than low temperature finishing. Onthe other hand, the finishing time required at high temperatures is lessthan that at low temperatures. Thus, the amount of initiator provided tothe system depends on how fast a finishing time is desired. Ordinarily,[residual monomer levels of 3-8 weight percent] the presence, underactivating conditions, of at least 5 x mole, preferably above l0 mole,of chemical initiator per 100 grams of copolymer will provide sufiicientfree radicals to reduce the residual monomer content over 12-32 fold [tobelow 0.25 percent] in an economically feasible time, e.g., about 20-50minutes.

In accomplishing thhe present finishing step, conditions should beavoided which tend to inhibit the creation of free radicals. Forexample, since polymerization of vinyl acetate is strongly inhibited byoxygen, the finishing step should be carried out under an atmosphereother than air, e.g. nitrogen or ethylene. The pH of an aqueous dispersion system further illustrates a potentiall adverse condition. Basicconditions seem to inhibit free radical formation in a hydrogenperoxide-zinc formaldehyde sulfoxylate reduc initiator system.Accordingly, acid conditions should be employed with this initiatorsystem. On the other hand, persulfate initiators can be used to providefree radicals when the system is basic.

Subject to the above paragraph, a variety of ingredients other than thecopolymer can be present in the system during the finishing step. Thus,if the copolymer is prepared in an aqueous dispersion or suspension,ingredients such as surfactants, thickeners, protective colloids,buffers, etc. will frequently be present. If solution polymerizationtechniques are employed, diluents or solvents can be present. Similarly,while the present invention has been thus far discussed in terms ofvinyl acetate/ethylene binary copolymers, it is also applicable in thepreparation of vinyl acetate/ethylene copolymers containing minoramounts of one or more other copolymerizable monomers. Thus, unsaturatedcarboxylic acids such as acrylic and methacrylic acid as well as otherderivatives, for example, substituted or unsubstituted esters of suchacids, as well as other vinyl compounds, e.g., vinyl chloride, vinylfluoride, etc. can be included in the vinyl acetate/ ethylene copolymerscontained in the system finished by the present process.

The present process is particularly useful in finishing systems whereinthe vinyl acetate/ethylene copolymer contains a post reactivecopolymerized monomer, e.g., glycidylacrylate, allyllalkyl] glycidylether, triallyl cyanurate, N-methylol acrylamide, etc. Since the presentfinishing process can be accomplished at low temperatures, the chancethat the post reactive monomer will prematurely react is diminished.

The following example illustrates the present invention. All parts areby weight unless otherwise indicated EXAMPLE The following ingredientsare charged into a stirred 5 gallon pressure vessel:

Grams Vmyl acetate 45-50 10% aqueous solution of Elvanol 5l05 polyvinylalcohol (88 hydrolyzedlow viscoslty) 2700 Water 2000 3% aqueous solutionof zinc-formaldehyde sulfoxylate 200 Feed 1 (comonomer) Grams Vmylacetate 3160 Feed 2 10% aqueous solution of Elvanol 51-05 polyvinylalcohol (88% hydrolyzedlow viscosity) 1800 3% aqueous solution ofzinc-formaldehyde sulfoxylate solution in Water 270 Water 200 AerosolMA- (sodium dihexylsulfosuccinate) 1 l0 Ethylene pressure is maintainedat 600 p.s.i., and, by addition of initiator solution, thepolymerization temperature is maintained at 60 C. The reaction exothermdies down after about 4 hours, at which time the ethylene pressure isreduced to atmospheric. At this point, the residual monomer content isabout 3%. Thereafter, 100 grams of the above-designated initiatorsolution are added to the emulsion. After maintaining the emulsion forabout 30 minutes at 40 C., the residual vinyl acetate monomer content isreduced by more than 85 [less than 0.35 percent, based on the weight ofthe copolymer].

In comparison, if the ethylene pressure is not reduced after theexotherm subsides or if additional initiator is not added to the reducedpressure system, an additional 30 minutes of reaction time reduces themonomer content only by about 17 percent [to only about 2.5 percent,based on the weight of the copolymer].

I claim:

1. In the process of preparing a copolymer system containing a vinylacetate/ethylene copolymer having 50- 95 weight percent copolymerizedvinyl acetate and a small level of residual vinyl acetate monomercomprising a main copolymerization step wherein a major portion of thevinyl acetate to be included in the copolymer [quantity of vinylacetate] is copolymerized with ethylene at a temperature of about30-150" C. and under an ethylene pressure of about 100-3000 p.s.i. untila substantial cessation of the reaction exotherm indicates thetermination of the main copolymerization step [to an extent such that atleast 0.35 percent residual vinyl acetate monomer, based on the weightof vinyl acetate/ethylene copolymer in the system, remains in thesystem] and a finishing step wherein the residual vinyl acetate monomercontent in the system is further reduced [to a level below 0.35percent;] the improvement comprising, accomplishing the finishing stepby reducing the pressure in the system to a value substantially belowthat at which the main copolymerization was accomplished and, while thesystem is under such reduced pressure, providing suflicientpolymerization initiating free radicals to the system to reduce theresidual vinyl acetate monomer content below the level at thetermination of the main copolymerization step [to an amount of less than0.35 percent].

2. The process of claim 1 wherein the pressure reduction in thefinishing step is to about that of atmospheric pressure.

3. The process of claim 2 wherein the copolymer system is an aqueousdispersion.

4. The process of claim 3 wherein, in the finishing step, polymerizationinitiating free radicals are provided by activating a chemicalinitiator,

5. The process of claim 4 wherein the chemical initiator is a peroxide.

6. The process of claim 5 wherein the peroxide is activated by areducing agent.

7. The process of claim 6 wherein the peroxide is hydrogen peroxide andthe reducing agent is zinc formaldehyde sulfoxylate.

8. The process of claim 4 wherein [3-8 weight percent, residual vinylacetate monomer remains in the system after the main copolymerizationstep and wherein] at least 5X10" mole of chemical initiator per 100grams of copolymer is provided to the system during the finishing step.

9. The process of claim 8 wherein the chemical initiator is a peroxide.

10. The process of claim 9 wherein the peroxide is activated by areducing agent.

11. The process of claim 10 wherein the peroxide is hydrogen peroxideand the reducing agent is zinc-formaldehyde sulfoxylate.

12. The process of claim 2 wherein the residual vinyl acetate monomercontent is reduced by more than percent during the finishing step [tobelow 0.25 percent].

13. The process of claim 12 wherein the copolymer system is an aqueousdispersion.

14. The process of claim 13 wherein, in the finishing step,polymerization initiating free radicals are provided by activating achemical initiator.

15. The process of claim 14 wherein the chemical initiator is aperoxide.

16. The process of claim 15 wherein the peroxide is activated by areducing agent.

17. The process of claim 16 wherein the peroxide is hydrogen peroxideand the reducing agent is zinc-tormal dehyde sulfoxylate.

References Cited The following references, cited by the Examiner, are

of record in the patented file of this patent or the original patent.

JOSEPH L. SCHOFER, Primary Examiner I. KlGI-IT III, Assistant Examiner

